The present invention relates to transmitters of the type used in the process control industry. More specifically, the invention relates to diagnostics for process control transmitters which include a resistance based temperature sensor.
Process control transmitters are used to monitor process variables in industrial processes. For example, a transmitter might monitor temperature and transmit such information back to a control room. Furthermore, some transmitters are capable of performing the control function directly. In order to monitor a process variable, the transmitter must include a sensor, for example, an RTD which is a resistance based temperature sensor.
As the RTD sensor ages or is subjected to harsh environmental conditions, the accuracy of the sensor tends to degrade. It is possible to compensate for this degradation by periodically recalibrating the transmitter. Typically, this requires an operator to enter the field and perform a calibration process on-site on the transmitter. This is both inconvenient and time consuming for the operator. Further, it is difficult to determine the condition of a sensor, prior to its ultimate failure.
It is also necessary for the sensors to be periodically replaced as they age. However, it is difficult to determine precisely when a replacement is necessary. Therefore, sensors are typically replaced well before their failure or, in some cases, they may fail unexpectedly.
A transmitter in a process control system includes a resistance-based sensor sensing a process variable and providing a sensor output. Self heating circuitry coupled to the sensor provides a self heating signal related to the sensor. Analog-to-digital conversion circuitry coupled to the sensor output provides a digitized sensor output, and transmitter output diagnostic correction circuitry provides an auto corrected output as a function of the self heating signal output or in another embodiment, the transmitter outputs a residual life estimate of the sensor as a function of the self heating index. A novel method of calculating the self heating index is also disclosed, for use in the present invention.